1. Usually used to express "love", or just "luv". It's usually less important than an actual "I love you", more like something you say at the end of an IM conversation to someone you don't necessarily "love". PS: the more 3's you add, the more you "love" the person.

2. I find that it also looks like a mouth to the side, like a kiss, the "3" being the lips. PS: the more 3's you add, the more kisses you're giving.

3. In other uses, one can also say that it means "less than three" or "smaller than three" but that is more mathematical.